1. Field of the Invention
The present invention relates to a control method, more particularly to a control method for controlling an asymmetric DC-DC converter.
2. Description of the Related Art
Generally, asymmetric DC-DC converters adopt complementary control techniques and are characterized by advantages such as low switching voltage stress and relatively simple structure. The asymmetric DC-DC converters, with the use of synchronous rectification techniques, are able to reduce loss attributed to rectification processes, thereby improving the conversion efficiency. The asymmetric DC-DC converters are therefore suitable for use at high frequencies.
Shown in FIG. 1 is a conventional asymmetric half-bridge DC-DC converter 900, which, in comparison with conventional symmetric converters, has first and second power switches Q1, Q2 driven in an alternating manner by respective signals that are complementary to each other. That is to say, when one of the first and second power switches Q1, Q2 is not driven, the other of the first and second power switches Q1, Q2 is driven. However, such an asymmetric complementary control is characterized by the drawback of presence of direct current biasing in the asymmetric half-bridge DC-DC converter 900. When loading of a load RL, increases, a biasing current that is generated by the asymmetric half-bridge DC-DC converter 900 increases. In order to reduce the impact of the biasing current, an inductance of an inductor Lm must be reduced, or larger magnetic cores (not shown) must be used. Such an approach, however, may have a significant adverse impact upon the conversion efficiency and may reduce density of the output power. On the other hand, although adopting an asymmetric turn ratio may reduce the biasing current, the ripple current of an output inductor Lo may consequently be increased. Under light loading, an inductor current that flows in a reverse direction will cause a voltage peak of a synchronous rectifier to increase, which reduces reliability of the asymmetric half-bridge DC-DC converter 900.
Therefore, how to implement an asymmetric DC-DC converter that is able to adapt to different loadings and that exhibits less biasing currents and ripple currents has been a subject of improvement in the industry.